In general, the plasma arc welding has a higher energy density than other welding such as a gas metal arc (GMA) welding and a gas tungsten arc (GTA) welding. Thus, the plasma arc welding can perform keyhole welding, i.e., can cause the plasma arc to penetrate from a front face (upper face) of a welding base metal (matrix, mother material) to a back face (lower face) while the welding is performed. If the keyhole welding is possible, the welding from the back face of the base metal is unnecessary, and therefore the welding work efficiency is significantly improved. During the keyhole welding, however, the keyhole tends to take an unstable behavior because of various factors, such as the temperature increase of the base metal during the welding, the atmosphere temperature, and magnetic blow caused by grounding. Therefore, high quality welding is only possible by an experienced person, and the automation is difficult.
To deal with this problem, for example, Patent Literature (Japanese Patent Application Laid-Open (Kokai) Publication No. 60-27473) uses a pulse current as the welding current, and causes a plasma jet to pulsate (fluctuate) so as to blow off the molten metal in the form of small droplet. This can form a weld bead without dripping (dropping). Patent Literature 2 (Japanese Patent Application Laid-Open (Kokai) Publication No. 8-39259) proposes a keyhole welding method that changes the plasma gas flow rate in a pulsating manner to avoid deficiencies such as burning through of a molten pool.